This invention relates to an optical transmitter and to a method for monitoring signal quality of a modulated optical signal output by a transmitter.
In optical communication systems, the demand for more bandwidth forces the increase in the data rates of the optical channels used in the backbone of optical communication systems. The increase of data rates has until now been achieved mainly by increasing the order of the modulation format of the optical channels. Upgrading the order of the modulation format for the optical channels has so far been straightforward due to the fact that only binary electrical driving signals have been utilized in optical transmitters of the communication system. In order to increase the spectral efficiency one can make use of multilevel modulation formats together with polarization division multiplexing such as 16 quadrature amplitude modulation 16QAM, 32 quadrature amplitude modulation 32QAM and even 64 quadrature amplitude modulation 64QAM. However, multilevel modulation requires larger optical signal-to-noise ratios and is more sensitive to nonlinear propagation impairments and laser phase noise. Further, the transmitter, having an optical modulator to generate the modulated optical signal and the respective modulation format, comprises transmitter components with nonlinear transfer functions so that pre-distortion is required. For example, a conventional optical transmitter can comprise Mach-Zehnder modulators MZM which have nonlinear transfer functions. This is illustrated with respect to FIGS. 1a, 1b. FIG. 1a shows the effect using a 2Vπ driving signal and FIG. 1b shows the effect using a 2Vπ driving signal with pre-distortion. According to FIG. 1, driving the Mach-Zehnder modulators at the optical transmitter with conventional 4PAM signals does not guarantee the generation of high-quality optical 16QAM signals so that some form of pre-distortion is required. This pre-distortion becomes even more critical when moving further towards higher-density modulation formats such as PDM-16QAM or PDM-64QAM. Furthermore, pre-distortion is beneficial for overcoming any penalties arising from the implementation of InP or Si photonic circuits in the optical system.
Accordingly, there is a need for effectively monitoring the signal quality of a modulated optical signal having a high order modulation format.